(1) Field of the Invention
The present invention relates generally to abrasives and abrasive tools suitable for surface grinding and polishing of hard and/or brittle materials. This invention more particularly relates to highly porous, bonded abrasive articles having an interconnected pore structure and methods for making same. The abrasives of this invention are useful in high performance grinding operations, such as backgrinding silicon, alumina titanium carbide and silicon carbide wafers, which are typically used in the manufacture of electronic components.
(2) Background Information
The use of porous abrasives to improve mechanical grinding processes is generally well known. Pores typically provide access to grinding fluids, such as coolants and lubricants, which tend to promote more efficient cutting, minimize metallurgical damage (e.g., surface burn), and maximize tool life. Pores also permit the clearance of material (e.g., chips or swarf) removed from an object being ground, which is important especially when the object being ground is relatively soft or when surface finish requirements are demanding (e.g., when backgrinding silicon wafers).
Previous attempts to fabricate abrasive articles and/or tools including porosity may generally be classified into one of two categories. In the first category, a pore structure is created by the addition of organic pore inducing media (such as ground walnut shells) into the abrasive article. These media thermally decompose upon firing, leaving voids or pores in the cured abrasive tool. Examples of this category are U.S. Pat. No. 5,221,294 to Carmen, et al., and U.S. Pat. No. 5,429,648 to Wu, and Japan Patents A-91-161273 to Grotoh, et al., A-91-281174 to Satoh, et al. In the second category, a pore structure may be created by the addition of closed cell materials, such as bubble alumina, into an abrasive article. See for example U.S. Pat. No. 5,203,886 to Sheldon, et al.
In an alternative approach, Wu et al., in U.S. Pat. Nos. 5,738,696 and 5,738,697, each of which is fully incorporated herein by reference, disclose an abrasive article and method for fabricating the same including fiber-like abrasive grains having a length to diameter aspect ration of at least 5:1. The poor packing characteristics of the elongated abrasive grains resulted in an abrasive article including increased porosity and permeability and suitable for relatively high-performance grinding.
As market demand has grown for precision components in products such as engines, refractory equipment, and electronic devices (e.g., silicon and silicon carbide wafers, magnetic heads, and display windows) the need has grown for improved abrasive tools for fine precision grinding and polishing of ceramics and other relatively hard and/or brittle materials. The abrasive tools known in the art have not proven entirely satisfactory in meeting the above stated needs. Therefore, there exists a need for improved abrasive articles and abrasive tools, and in particular, those including a relatively high degree of porosity.
One aspect of the present invention includes a method for fabricating an abrasive article. The method includes blending a mixture of abrasive grain, bond material, and dispersoid particles, the mixture including from about 0.5 to about 25 volume percent abrasive grain, from about 19.5 to about 49.5 volume percent bond material, and from about 50 to about 80 volume percent dispersoid particles. The method further includes pressing the mixture into an abrasive laden composite, thermally processing the composite, and immersing the composite into a solvent for a period of time suitable to dissolve substantially all of the dispersoid, the dispersoid being soluble in the solvent. Further, the abrasive grain and the bond material are substantially insoluble in the solvent. In one variation of this aspect, the bond material includes from about 35 to about 85 weight percent copper, from about 15 to about 65 weight percent tin, and from about 0.2 to about 1.0 weight percent phosphorus. In a further variation of this aspect, the dispersoid includes granular sodium chloride and the solvent includes boiling water.
In another aspect, this invention includes an abrasive segment for a segmented grinding wheel. The abrasive segment includes a composite including a plurality of superabrasive grains and a metal bond matrix sintered together at a temperature ranging from about 370 to about 795xc2x0 C., the composite having a plurality of interconnected pores disposed therein, the composite including from about 0.5 to about 25 volume percent abrasive grain, from about 19.5 to about 49.5 percent metal bond and from about 50 to about 80 volume percent interconnected porosity. The metal bond matrix includes from about 35 to about 70 weight percent copper, from about 30 to about 65 weight percent tin, and from about 0.2 to about 1.0 weight percent phosphorus. The plurality of superabrasive grains is selected from the group consisting of diamond and cubic boron nitride, the superabrasive grains having an average particle size of less than about 300 microns.
In a further aspect, this invention includes a segmented grinding wheel. The grinding wheel includes a core having a minimum specific strength of 2.4 MPa-cm3/g, a core density of 0.5 to 8.0 g/cm3, and a circular.perimeter. The grinding wheel further includes an abrasive rim including a plurality of segments, each of the segments including a composite having a plurality of abrasive grains and a metal bond matrix sintered together at a temperature ranging from about 370 to about 795xc2x0 C., the composite having a plurality of interconnected pores disposed therein, the composite including from about 50 to about 80 volume percent interconnected porosity. The grinding wheel still further includes a thermally stable bond between said core and each of said plurality of segments.
In still a further aspect, this invention includes a method for fabricating an abrasive article having from about 40 to about 80 volume percent interconnected porosity. The method includes blending a mixture of abrasive grain, organic or other non-metallic bond material, and dispersoid particles, the mixture including from about 0.5 to about 25 volume percent abrasive grain, from about 19.5 to about 65 volume percent organic bond material, and from about 40 to about 80 volume percent dispersoid particles. The method further includes pressing the mixture into an abrasive laden composite, thermally processing the composite, immersing the composite into a solvent for a period of time suitable to dissolve substantially all of the dispersoid, the dispersoid being soluble in the solvent. In a variation of this aspect the dispersoid includes granular sugar and the solvent includes boiling water.
In yet another aspect, this invention includes an abrasive segment for a segmented grinding wheel. The abrasive segment includes a composite including a plurality of superabrasive grains and a non-metallic bond matrix cured together, the composite having a plurality of interconnected pores disposed therein and including from about 0.5 to about 25 volume percent abrasive grain, from about 19.5 to about 65 percent non-metallic bond and from about 40 to about 80 volume percent interconnected porosity. The plurality of superabrasive grains are selected from the group consisting of diamond and cubic boron nitride, the plurality of superabrasive grains having an average particle size of less than about 300 microns.
In yet a further aspect, this invention includes a segmented grinding wheel. The grinding wheel includes a core having a minimum specific strength of 2.4 MPa-cm3/g, a core density of 0.5 to 8.0 g/cm3, and a circular perimeter. The grinding wheel further includes an abrasive rim including a plurality of segments, each of the segments including a composite of abrasive grains and a non-metallic bond matrix cured together, the composite having a plurality of interconnected pores disposed therein and including from about 40 to about 80 volume percent interconnected porosity. The grinding wheel still further includes a thermally stable bond between the core and each of the plurality of segments.